The present invention relates to a coated cutting insert for chipforming machining.
The Chemical Vapor Deposition (CVD) of alumina on cutting tools has been an industrial practice for more than 15 years. The wear properties of Al.sub.2 O.sub.3 as well as of TiC and TiN have been discussed extensively in the literature.
The CVD technique has also been used to produce coatings of other metal oxides, carbides and nitrides with the metal selected from the transition metals of the groups IVB, VB and VIB of the Periodic Table, silicon, boron and aluminum. Many of these compounds have found practical applications as wear resistant or protective coatings, but few have received as much attention as TiC, TiN and Al.sub.2 O.sub.3.
Initially, coated tools were intended for turning applications, but today tools designed for milling as well as drilling applications, are coated. Improvements of the bonding to the substrate (generally a cemented carbide) and between different coating materials have resulted in a plurality of coating combinations with double-, triple- and multi-layer structures.
The reaction mechanisms occurring during the CVD of Al.sub.2 O.sub.3 have been analyzed, but little has been mentioned about the stability and microstructure of the deposited Al.sub.2 O.sub.3 phases and how the formation of these phases depends on the deposition process.
Al.sub.2 O.sub.3 crystallizes in several different phases of which the alpha-structure (corundum) is the thermodynamically stable phase at typical deposition temperatures. The metastable kappa-phase is the second most commonly occurring modification in CVD-Al.sub.2 O.sub.3. Other infrequently occurring types are theta-, gamma- and delta-Al.sub.2 O.sub.3.
In commercial tools, Al.sub.2 O.sub.3 is always applied on TiC-coated cemented carbide (see, e.g., U.S. Pat. No. 3,837,896, now U.S. Pat. No. Re. 29,420) and therefore the interface reactions of the TiC-surface are of particular importance. The TiC-layer should also be understood to include those layers having the formula TiC.sub.x N.sub.y O.sub.z in which the C in TiC is completely or partly substituted by oxygen and/or nitrogen. In the system TiC.sub.x N.sub.y O.sub.x there is 100% miscibility. There can be one or more layers of this kind. Similar relationships also exist within other systems, e.g., Zr--C--N--O.
The purpose of the present invention has been to obtain such an Al.sub.2 O.sub.3 -layer with the correct crystallography, microstructure and morphology and under nucleation conditions such that the desired Al.sub.2 O.sub.3 -phases will be stabilized.
A typical surface structure of a CVD-Al.sub.2 O.sub.3 coating contains coarse-grained islands of alpha-Al.sub.2 O.sub.3, the first Al.sub.2 O.sub.3 phase to nucleate, surrounded by much more fine-grained kappa-Al.sub.2 O.sub.3 areas. When greater amounts of such alpha-Al.sub.2 O.sub.3 exist the individual islands join together. The ratio of coarse-grained/fine-grained areas can vary over a broad range.
Also, the alpha/kappa ratio measured by X-ray diffraction will vary over a wide range. A higher ratio is obtained at long coating times and after further heat treatment. After about 3 hours heat treatment at 1000.degree. C., essentially all the thermodynamically metastable kappa-Al.sub.2 O.sub.3 is transformed to stable alpha-Al.sub.2 O.sub.3. It is important to note that the transformation kappa.fwdarw.alpha takes place without any great changes in surface morphology. The alpha-Al.sub.2 O.sub.3 formed from kappa-Al.sub.2 O.sub.3 is fine-grained.
The kinetics behind the phase transformation of metastable kappa-Al.sub.2 O.sub.3 to stable alpha-Al.sub.2 O.sub.3 is not clear. It should be noted, however, that only small rearrangements of the close-packed oxygen layers that are common to both structures, are needed to convert kappa- to alpha-Al.sub.2 O.sub.3.
In U.S. Pat. No. 4,180,400 (now U.S. Pat. No. Re. 31,526) a method of making kappa-Al.sub.2 O.sub.3 has been proposed being based essentially on the fact that an addition of TiCl.sub.4 to the gas mixture results in kappa-Al.sub.2 O.sub.3 formation. Any kappa-phase, formed under these conditions, does not have an epitaxial relationship with the layer below a thick Al.sub.2 O.sub.3 -layer, however. (A thick Al.sub.2 O.sub.3 -layer is more than 2 .mu.m Al.sub.2 O.sub.3).
The microstructure of an alpha-Al.sub.2 O.sub.3 coating initially nucleated as alpha-Al.sub.2 O.sub.3 is characterized by a grain size of about 0.5-5 .mu.m, preferably 0.5-2 .mu.m, and the presence of pores. Contrary to this, a kappa-Al.sub.2 O.sub.3 coating is essentially pore-free and has a much smaller grain size of about 0.5-2 .mu.m, preferably 0.05-1 .mu.m, usually about 0.2-0.5 .mu.m.